In the drilling of wells into subterranean earth formation, air is sometimes used as the drilling fluid to flush cuttings from the bottom of the hole, as drilling progresses. If gas or oil is produced from the earth and mixes with the air in sufficient quantity to form a combustible mixture, a downhole fire can result.
In U.S. Pat. No. 3,845,784, granted Nov. 5, 1974, there is disclosed a so-called float valve which permits circulation of drilling fluid or air but automatically closes to prevent reverse flow, but such a float is useless to prevent the continued flow of air to the fire or to warn of the existence of a fire in the borehole.
In U.S. Pat. No. 3,537,518 granted Nov. 3, 1970, there is disclosed a "Safety Drill Pipe Float Valve With Heat Responsive Shutoff Sleeve" adapted to be installed in the drill pipe string above the bit, wherein a valve seat is formed as part of a member which is held in an initial position by a bond of solder or babbitt, which is heat sensitive so as to be liquified in the event of a borehole fire and allow the seating sleeve to engage a valve head and prevent the further flow of air through the drill string.
It may occur, however, that in the event of fire, damage may also be caused to the drill collars or drill string members spaced substantially upward in the hole from the bit, sometimes without leaving visible evidence. Therefore, following the downhole fire, it is possible that if each of the pipe members above the bit is not tested, a member or collar may fail during drilling of another well, resulting in additional problems and expenses.